1. Field of the Invention
The present invention relates to cellular networks. More specifically, a method for performing an Inter-RAT handover is disclosed.
2. Description of the Prior Art
Inter system handovers between Global System for Mobile Communications (GSM) and Universal Terrestrial Radio Access Network (UTRAN) are expected to play an increasingly important role, as many 3rd Generation (3G) operators will provide wide-area coverage with the GSM Base Station System (BSS) and use the 3G UTRAN in relatively small traffic hotspots. A dual-system user equipment (UE) is capable of supporting both GSM and UTRAN systems. Each time such a UE moves between the two types of coverage areas, an inter system handover must be performed. Because the 3GPP specifications make wide use of the term “radio access technology (RAT)” when referring to application network technologies, the inter system handover is often called an inter-RAT handover. An inter-RAT handover to UTRAN procedure hands the UE over from a GSM system to a UTRAN system. Similarly, an inter-RAT handover from UTAN procedure hands the UE over from the UTRAN system to a GSM system.
To perform an inter-RAT handover to UTRAN, a large amount of information, such as spreading, channelization codes and frequency information, must be provided to the UE. Relaying so much information to the UE using an extended (and possibly segmented) GSM handover command would be impractical. In particular, segmentation over more than two GSM air interface messages would have an unacceptable impact on handover performance. Consequently, the use of predefined and default Universal Mobile Telecommunications System (UMTS) radio configurations was proposed. This is discussed in detail in the specifications 3GPP 25.331 “Radio Resource Control (RRC) Protocol Specification”, and 3GPP TS 25.922 “Radio Resource Management Strategies”, both of which are included herein by reference. Attention is drawn in particular to clause 5.1.5.2 of 3GPP TS 25.922, which deals with the use of predefined radio configuration during handover from 2G (i.e., GSM) to 3G (i.e., UTRAN). In short, by utilizing default configurations stored in every UE, and up to sixteen predefined configurations that are downloadable from the UTRAN, it is possible to greatly reduce the total amount of information that needs to be sent to the UE to perform the inter-RAT handover to UTRAN procedure, and thus ensure that the handover performance is acceptable.
The UE can support both circuit switched (CS) and packet switched (PS) connections, and both types of connections must be handed over. The handover to UTRAN procedure is quite complex, and a complete description of every aspect of the procedure is quite beyond this background description. However, for more details, in addition to the above-noted specifications, attention is also drawn to 3GPP TS 23.060 “General Packet Radio Service (GPRS); Service description; Stage 2”, and 3GPP TS 24.008 “Mobile radio interface layer 3 specification; Core Network Protocols; Stage 3”, both of which are included herein by reference. Of particular note is clause 6.13.2.2 of 3GPP TS 23.060, which details aspects of the handover to UTRAN procedure for PS services; clause 4.7.1.7 of 3GPP TS 24.008, and, within 3GPP TS 25.331, clauses B.6.2, 8.3.6 and 10.2.16a.
Briefly, though, the above specifications indicate that both PS and CS services are to be handed over concurrently. As it turns out, though, the handover to UTRAN procedure can only utilize the default/predefined UMTS radio configurations for the CS connection, and consequently a great deal of information must be explicitly provided for the PS connection. As indicated above, this can lead to segmentation over more than two GSM air interface messages, which will adversely impact the handover performance. In addition, the above references do not provide a clear method for enabling the UE to determine whether or not a signaling connection exists after the handover to UTRAN procedure. This may cause the UE to subsequently use an incorrect procedure (initial direct transfer or uplink direct transfer) to transfer a message to the UTRAN.